This invention relates to spectroscopic analysis of chemical samples for the presence of, or characteristics of, one or more specified molecules.
Cavity ring down spectroscopy (CRDS) is a laser-based, high sensitivity, absorption measurement technique that has become competitive with alternative techniques, such as intra-cavity laser absorption spectroscopy, frequency modulation spectroscopy, multipass spectroscopy and photoacoustic spectroscopy, for performing spectroscopic analysis of a sample. CRDS exploits the properties of a high finesse optical resonator, usually formed by two or more high reflectivity mirrors defining an optical cavity, with a sample located in the optical cavity. A narrow bandwidth laser light beam is injected into the optical resonator and abruptly terminated. The resulting rate of decay, R, of light beam intensity is measured and is linearly related to the optical resonator losses by a relation
R=1/xcfx84=Ltot/xcex94trt,xe2x80x83xe2x80x83(1)
Ltot=L(refl)+L(sample).xe2x80x83xe2x80x83(2)
Here xcfx84 is the ring down decay constant (sec), xcex94trt is the optical cavity round trip time, and L(refl) and L(sample) are measures of the light beam intensity losses due to the resonator alone (no sample present) and due to sample absorption and scattering, respectively, at the chosen wavelength.
In the past, CRDS has been primarily applied to gas phase samples, because of the relative simplicity of such samples. For a typical gas sample, an absorption spectral scan of cavity losses with no sample present is first performed to serve as a reference. Subsequently, the sample gas is injected into and substantially fills the cavity, and total sample absorption spectrum is again measured. The final quantitative measurement of the gas absorption is determined by subtracting the reference spectrum from the total spectrum. Because a gas fills the optical cavity uniformly, the light beam circulating within the cavity will be substantially uniformly absorbed along the entire round trip route along the resonator. Thus, no corrections need be made for the shape of the light beam within the optical cavity or for the unknown length of the sample, in order to obtain a fully quantitative measurement. In certain instances, the optical resonator, also referred to as a ring down cavity (RDC), encloses a sample having a finite length that is shorter than the length of the RDC. In particular, a flame having a known geometry has been studied for its composition, temperature and species profile(s) using CRDS.
Unless the sample gas contains particulates, such as dust, the gas will produce negligible scattering of the light beam within the RDC. Further, a gas that fills the cavity has no reflecting facets produced by a physical interface between the sample and the remainder (if any) of the volume within the RDC that can interfere with the light beam.
For a condensed phase sample, such as a liquid, thin film or bulk solid, or a gas that does not fill the cavity, however, these problems are present and would appear to preclude use of CRDS as an absorption measurement tool. If a condensed phase or gas sample is positioned at random within, but does not fill, a high finesse optical resonator that includes two or more mirrors and contains a gas or a vacuum, one or more physical interfaces between the sample and the remainder of the RDC volume is created. The interface(s) can produce reflection losses inside the RDC that far exceed the other losses associated with the RDC and would compromise the observation of a single exponential decay of light beam intensity at the RDC output terminal, by formation of coupled resonators. Because light beam intensity losses per unit length within a condensed phase sample tend to be large, filling the RDC with this sample would severely degrade the resonator finesse and thus interfere with a straightforward measurement of the ring-down signal and reduce sensitivity to different samples. If the round trip length of the resonator is reduced in order to minimize the sample length, the decay constant for, and sensitivity to, the sample within the RDC would be correspondingly reduced.
For a non-cavity-filling sample, the sample length within the RDC must be chosen so that the total sample losses are comparable to the RDC optical losses, and so that the ring-down decay constant does not become too small for accurate detection by the available electronics. One approach to this problem is to probe the sample within the RDC with an evanescent wave that is external to the RDC, as proposed by Pipino et al [U.S. Pat. No. 5,943,136]. In one embodiment of the Pipino et al approach, the RDC is a monolithic ring cavity, wherein the injected radiation is reflected by total internal reflection, thereby generating at each facet an evanescent wave whose path length within the sample is determined by the light beam wavelength, by the RDC refractive indices and by the nature of the sample. In an alternative embodiment, a Pelin-Broca prism is positioned within the RDC so that a light beam reflected at a prism facet produces a single evanescent wave that propagates within the sample. In both embodiments, the effective sample path length is determined by the light beam penetration depth, rather than by the sample round trip length within the RDC. The Pipino et al approach could have advantages in certain applications, but this approach can severely limit the sample length probed for a poorly absorbing sample, or for small concentrations of a sample that is sparsely distributed within a non-absorbing matrix. Typically, the effective sample penetration depth in evanescent wave CRDS is on the order of one-tenth of the radiation wavelength, or about 70 nm to 1 xcexcm for radiation wavelength in the near infrared to mid-infrared region.
What is needed is a spectroscopic testing system that provides preparation and accurate testing for the presence of one or more target molecules that may be present in an amount ranging from an extremely small trace to a modest percentage of the sample contents, that can be used to test for the presence of one or more different molecules, that can provide confirmation or refutation of the postulated presence of one or more target molecules in as little as a few minutes, that can be quickly expanded to cover testing for other molecular markers, based on the initial test results, and that does not require performance of various wet chemistry procedures to determine whether a particular target molecule is present. Preferably, the system should have very small losses of the probe light beam. Preferably, the system should be small enough to located in the on-site office of test personnel, such as a medical doctor, a veterinarian, an industrial health and safety officer and the like. Preferably, the system should allow preservation of the sample(s) for further and possibly more elaborate testing at another time.
These needs are met by the invention, which uses: a resonant optical cavity, configured to perform in a cavity ring down spectroscopy (CRDS) mode and (partially) filled with a non-cavity-filling sample at a selected temperature and pressure; an intense collimated source of narrow band coherent light (preferably polarized) with a selectable wavelength; a light coupler to couple a light beam into and out of the cavity; and an absorption analyzer to receive the light coupled out of the cavity, to determine the absorption by the sample and the absorption due to cavity losses, and to provide a graphical or other representation of the sample absorption versus wavelength. The sample may have the form of a gas, a liquid, a thin film, a solid that is not in the form of a thin film (referred to herein as a xe2x80x9cbulk solidxe2x80x9d) and any other form that does not completely fill the cavity.
In a preferred embodiment, two or more highly reflective, spaced apart mirrors are arranged to form an optically stable cavity. The sample, and any necessary sample support, has at least two substantially parallel interfaces with another selected medium (preferably, but not necessarily, a vacuum) within the cavity and is arranged so that each of these parallel interfaces intercepts the propagating light beam at an incidence angle equal to the Brewster angle for that interface and for the wavelength used. This arrangement ensures that substantially all of a p-wave polarization component (parallel to a plane of incidence) of the light beam is transmitted into and out of the sample, with little or no light beam reflection at either sample/medium interface. The light beam is introduced into the optical cavity, makes two or more round trips through the optical cavity and through the sample, is extracted from the cavity, and is analyzed to estimate the absorption losses within the sample. This analysis is performed at one or more selected wavelengths, and the resulting absorption spectrum is used to determine one or more characteristics of the sample, such as presence of, or concentration of, a target molecule within the sample.